Nested ParallelFor

How do I parallelize the following code using ParallelFor:

`const TMap < int32, TSet < int32 > > map;

for(int i=0;i< map.Num();++i)
{
for (int j=0;j< map[i].Num();++j)
{
//do something
}
}`

using ParallelFor on the outer loop, makes sense, i am not sure about nested ParallelFor giving any performance gain